Radial turbocharger turbines are provided with diffusers extending axially downstream from a turbine wheel around a wheel centerline. The diffuser is configured to reduce airflow velocity. To accomplish this, the cross-sectional area of the diffuser increases from an upstream end of the diffuser to the downstream end of the diffuser. Because the mass flow rate through the diffuser is constant, this increased cross-sectional area provides for decreased velocity, decreased dynamic pressure, and increased static pressure.
Turbines are designed for optimal operation at a design operating condition. At this operating condition, an exhaust gas stream will enter the diffuser having substantially axial flow. Nevertheless, at so called ‘off design’ operating conditions (i.e., substantially different operating conditions than the design operating condition), the exhaust gas stream may be characterized by high exit swirl, which can be in either direction depending on the particular operating condition. These off design operating conditions occur frequently in turbocharger turbine operation due to pulsing inlet flow conditions, size constraints (e.g., a turbine diameter smaller than optimum), and conditions where variable nozzle guide vanes (in the turbine inlet) are operated away from their nominal position. These off design operating conditions result in a very low blade speed ratio condition for the turbine stage which results in a high degree of exit swirl.
Thus, in these off design operating conditions, while the downstream (i.e., axial portion of the) velocity of the exhaust gas stream passing through the diffuser is reduced, it will still have a significant tangential (i.e., circumferential) velocity portion (i.e., a circular motion around the axial direction). This tangential velocity portion, in combination with the axial velocity portion, creates a swirling (i.e., spiraling) exhaust gas stream. This tangential velocity portion increases the total kinetic energy of the exhaust stream over that of just the axial velocity portion, and thereby causes efficiency losses across the off design operating range of the turbine, particularly under transient conditions.
It should be noted that any effort to deal with this problem is constrained by size limitations for the turbocharger, i.e., package constraints. This is particularly true for automotive turbochargers, which typically have significant size limitations.
A traditional conical diffuser design can be very efficient when dealing with zero or low levels of inlet swirl. Nevertheless, applying a traditional conical diffuser design to a swirling flow with package constraints typically results in significant separation of the flow and inefficient diffusion. De-swirl vanes with their leading edge angle matched to the flow swirl angle can be used in attempt to manage the problem, but these only operate effectively at a small range of operating conditions because the swirl angle of the flow into the diffuser varies dramatically across the useful operating range of the turbine. Thus, the use of de-swirl vanes results in high (angle of) incidence losses into the de-swirl vanes at operating conditions that are not close to the operating conditions for which the vanes were designed.
Accordingly, there has existed a need for an automotive turbocharger turbine diffuser that is both compact and highly efficient in reducing the kinetic energy of an exhaust gas stream.